1. Field of the Invention
The present invention relates to bulk devices and electronic scanning or switching techniques for efficiently steering antenna beams for transmission and reception of microwave and millimeter wave frequencies, and more particularly to squint-free electronic beam steering for antenna systems with a large aperture "fill-up time" or large gain-bandwidth product.
2. Description of the Related Art
Phased array antennas have numerous advantages in the transmission and reception of signals for radar, communication, and free space data links. Electronic scanning of one or more simultaneous antenna beams is an implied property of the modem phased array antenna system. True electronic scanning means no physical movement by the antenna or any of its component parts to accomplish almost instantaneous movement of the antenna beam upon command.
The installation ease is the first special advantage of no mechanical movement. Instead of a large radome housing the antenna of a ground radar, the phased array antenna can be incorporated into the roof and sides of buildings. Instead of a small dish antenna surrounded by a 2-axis or 3-axis mechanical gimbal and its required angular rotation clearance space consuming the entire nose of an aircraft a flush-mounted array could be incorporated into the skin of an aircraft. Instead of placing a mechanically complex unfurlable reflector antenna and its positioner on a spacecraft, a phased array antenna allows mechanically simple, fixed, compact structural panel with a scanning beam without a gimbal.
Near instantaneous, inertialless electronic movement of the antenna beam is the second special advantage of the phased array antenna. This near instantaneous electronic movement of the antenna beam is especially useful for maneuvering aircraft maintaining multiple communication links with a remote locations or multiple nearby maneuvering aircraft. This same advantage is also useful for airborne radar in performing ground mapping, terrain avoidance, obstacle avoidance, and SAR imaging while maneuvering.
Ultra low sidelobe antenna patterns producible by a phased array antenna are a significant performance advantage for radar clutter reduction, interference reduction, and reduction in vulnerability to jammers. In a related category is the phased array's ability to steer antenna pattern nulls towards the origin of interfering or jamming signals. These important features give the radar or communication link dramatically improved figure of merit in the rejection of clutter and man-made interference.
Current technology is rapidly producing the ability to use extremely wideband signal waveforms, such as Wake, D., "A 1550-nm Millimeter-Wave Photodetector with a Bandwidth-Efficiency Product of 2.4 THz," Journal of Lightwave Tech., Vol. 10, No. 9, July 1992, 908-912. The use of these extremely wideband signal waveforms gives the opportunity of emission security and frequency reuse as well as performing functions requiring increased data rates.
The gain-bandwidth product limitation of a phase-only steered phased array antenna is most easily characterized by beam squint. Beam squint is the movement of the phase-only steered main antenna beam towards its only potentially wideband coherent antenna beam position, usually broadside as the frequency increases. Frank, J., "Bandwidth Criteria for Phased Array Antennas," Phased-Array Antennas, Oliner, Arthur A., and Knittel, George H., Editors, Artech House, Inc., Dedham, Mass. 1972. p 243-253 calculates the bandwidth of this effect by frequencies for which the beam response has degraded by 3 dB. from the peak. This bandwidth can also be related to the "Fill-up" time or time duration of a pulse necessary to simultaneously illuminate the entire antenna aperture by an incident pulse from a worst case direction. Failure to simultaneously illuminate the entire antenna aperture, prevents a phase-only steered array from achieving its full coherent gain.
Squint-free beamsteering is achieved by replacing phase steering with true time delay (TTD) steering together with a dispersionless beamforming network (BFN). At the antenna beam centers of squint-free beamsteering, the TTD beamsteering devices will introduce no frequency distortions on the signals. The scanning array control for TTD beamsteering is very similar in form to the standard array formulas, such as Elliott, Robert S., Antenna Theory and Design, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1981, p128 by the uniform progressive steering phase factor, .alpha..sub.z, in the array factor A.sub.a (.theta.): ##EQU1## where: I.sub.n is the amplitude of array element n=0,1, . . . ,N
w=exp[j(kd cos.theta.-.alpha..sub.z)], PA1 .theta. is the polar angle of the linear array PA1 k is the free space propagation constant, PA1 d is the array lattice distance between elements. PA1 where: z=exp[s(d cos.theta.-f.sub.z)/c], PA1 s=.sigma.+j.omega.=complex frequency, PA1 c=velocity of light, PA1 f.sub.z =progressive true time delay steering factor.
It can be shown that the above summation formula may be expanded and generalized using progressive true time delay steering instead of phase steering and the complex frequency, s, to: EQU A.sub.a (z)=[I.sub.N /I.sub.0 ][z.sup.N +(I.sub.n-1 /I.sub.N)z.sup.N-1 +. . . +(I.sub.1 /I.sub.N)z+I.sub.0 /I.sub.N ]
In the A.sub.a (z) array factor form, the z variable satisfies the time delay variable of the z-transform. By constructing beamsteering true time delay lines for each array element according to this formula, the antenna scanning coherence center of this array is located at the coherence center of the I.sub.0 array element. Similarly, by constructing the beamsteering true time delay lines for z.sup.-N A.sub.a (z), the antenna scanning coherence center of this array is located at the coherence center of the I.sub.N array element.
There are several microsecond-speed photonic approaches in the literature for overcoming phase only steering: Discrete beam positions, and hybrid phase/time-delay sub-arraying. Cardone, L., "Ultra-Wideband Microwave Beamforming Technique," Microwave Journal, April 1985, p 121-128 gives an example of the discrete beam position technique with optical fibers in an optically coherent BFN (beamforming network). He does not include the switching technique for electronically selecting among the plurality of discrete antenna beams and we are not yet able to maintain the necessary optical tolerances for the optically coherent BFN. The second electronic concept for avoiding phase-only steering is to use the hybrid phase/time-delay steering wherein the phase steering is used within a sub-array whose size is limited by bandwidth and to use time-delay steering among the subarrays to maintain the sub-array bandwidth over the physically larger array. Ng, W., Walston, A. A., Tangonan, G. L., Lee, J. J., Newberg, I. L., & Bernstein, N., "The First Demonstration of an Optically Steered Microwave Phased Array Antenna Using True-Time-Delay," J. of Lightwave Technology, Vol. 9, No.9, p 1124-1131, September 1991 implement this concept using a switch with large numbers of active devices per unit shifter. Subarray systems with hybrid phase/time-delay scanning off broadside fall subject to large gains losses and higher sidelobes as bandwidth increases, scan angle increases, or beamwidth decreases, according to Tang, R., "Survey of Time-Delay Beam Steering Techniques," Phased Array Antennas, Oliner, Arthur A., and Knittel, George H., Editors, Artech House, Inc., Dedham, Mass. 1972. p 254-260.
The required time shifter is physically not significantly different from recent embodiments of a phase shifter used in microstrip circuits. A preferred microstrip method of implementing a microwave phase shifter is to use diode switched passive delay lines, potentially yielding the differential time delay which is necessary for wideband beam steering. Typically in order to reduce cost of an expensive component, the steering element is designed to be multiply reused. The beam steering computer calculates the phase required for a single specific frequency to determine which delay lines to select by switching. The multiple reuse is accomplished by the application of modulo 2.pi. radian arithmetic which allows the computer to select a delay line which is shorter than the true time delay length by an integral number Of wavelengths for a specific single frequency. This re-used switched delay line shifter introduces a frequency dispersion without the advantage of squint-free beamsteering of the antenna beam.
Cost is the driver for requiting multiple reuse of components of a steering element of a phased array. The first reuse reason is the potential of component state reuse. The phase shifter accomplishes phase state reuse by modulo 2.pi. radian arithmetic. Many phase states can be reused on a single frequency basis. This means that for identical shifters, many more true time delay states are required than for identical phase shifting states. The second reuse reason is that microwave delay space is a precious resource according to Thompson, James D., U.S. Pat. No. 5,012,254, Apr. 30, 1991. If each array element requires its own shifting module to be located near or within the aperture of the antenna and a close spaced grating-lobe-free aperture is desired, significant aperture depth is required, often making conformal or tiled antenna structures dimensionally not possible without significant performance compromise. The third reuse reason is that if N.sup.2 phase/time shifters of an N by N element array are required, this is a significant total cost. Making all N.sup.2 phase/time shifters identical may make an affordable unit production cost. The fourth reuse reason is for the special cases of circular and conformal phased array beam steering. "The rotation of tapered excitations usually requires control of both amplitude and phase and this represents one of the limitations of beam cophasel systems due to the cost and complexity of Scanning", according to Davies, D. E. N., "Circular Arrays," The Handbook of Antenna Design, A. W. Rudge, K. Milne, A. D. Olver, P. Knight, Editors, Chapter 12, Peter Perigrinius, Ltd., 1986, p 992-1023. The classic choices are to scan circular ting arrays using weighted phase modes or to scan subarrays using phase shifters within a Butler matrix. Unfortunately, extremely wideband true phase shifters even more difficult to fabricate than extremely wideband than true time delay shifters and typically the phase shifter methods do not always provide for independent amplitude taper control.
In view of the above a need is apparent for an improved, preferable optical, squint-free beamsteering device for a phase array antenna. It is also imperative for extremely wideband use, that squint-free beamsteering must be implemented in a manner which dramatically reduces the the number of time shifters required. Further, if monopulse angle tracking schemes are to be used, the extreme change in the antenna beamwidth must be suppressed to obtain a frequency independent monopulse indicated spatial angle.
It is therefore an object of this invention to provide a frequency independent beamsteering and beamforming network that: (i) provides increased microwave isolation, reduced volume, weight, and part count of the beamsteering elements, (2) uses optical time delay lines to minimize space and weight resources, (3) uses low loss receiving optical beamforming networks, (4) reuses the time delay shifter for additional array elements rather than reuses the delay length in a non-frequency independent manner, (5) provides simplified computation of beamsteering and beam shaping commands for conformal arrays, (6) provides reduced number of commanded states to achieve 2-dimensional beamsteering, (7) uses squint-free beamsteering of the antenna beam center, (8) uses a unique antenna true time delay antenna scanning coherence center, (9) uses simultaneous multiple antenna beams, (10) provides a mechanism for minimizing beamwidth variation with frequency, (11) provides extremely wideband monopulse angle sensing, and (12) provides microsecond beamsteering.